There is known, as the above-described hybrid vehicle, a one-motor two-clutch parallel hybrid vehicle, for example, described in Japanese Unexamined Patent Application Publication No. 2010-179865. In this hybrid vehicle, a motor/generator is coupled and arranged between an engine and driving wheels, a first clutch can connect and disconnect the engine with and from the motor/generator, and a second clutch can connect and disconnect the motor/generator with and from the driving wheels.
This hybrid vehicle can select an EV mode for electric (EV) travel with the motor/generator if the first clutch is disengaged and the second clutch is engaged, and a HEV mode for hybrid (HEV) travel with the cooperation of the motor/generator and the engine if both the first and second clutches are engaged.
In the one-motor two-clutch parallel hybrid vehicle, if a request driving force is increased, for example, because an accelerator pedal is depressed during travel in the EV mode that is selected in a small-load and low-rotation-speed situation and the request driving force is no longer provided only by the motor/generator, the mode is changed to the HEV travel (the HEV mode) with the cooperation of the motor/generator and the engine.
To start the engine for changing the mode from the EV mode to the HEV mode, the first clutch, which is in the disengaged state in the EV mode, is engaged, and the engine is started by a motor torque from the motor/generator. However, to ensure start-up of the engine as described in Japanese Unexamined Patent Application Publication No. 2010-179865, the rated torque of the motor/generator has to cover an electric (EV) travel torque part that is the sum of a driving torque part corresponding to a running resistance (an air resistance, a rolling resistance, etc.) of the vehicle and an acceleration torque margin part corresponding to a predetermined acceleration margin part of the vehicle, and an engine start torque (cranking torque) part.
Meanwhile, the motor/generator is desired to have a rated torque as small as possible and is desired to be downsized in view of the cost and mounting space of the motor/generator. Thus, the amplitude of the rated torque is restricted. However, although the rated torque of the motor/generator is restricted, the required acceleration torque margin part and engine start torque part are almost determined, and these margin part and torque part cannot be decreased. Hence, the driving torque part obtained by subtracting the acceleration torque margin part and engine start torque part from the rated torque of the motor/generator becomes small.
If the driving torque part becomes small, the travel in the EV mode only with the motor/generator becomes unavailable at an early timing (even at a low vehicle speed) because of insufficiency of the torque. Thus, the mode has to be changed to the HEV mode with the cooperation of the engine and the motor/generator at an early timing (even at a low vehicle speed). This may narrow an EV mode range that provides an improvement in specific fuel consumption. Hence, fuel efficiency of the vehicle may be degraded.